A cyanide process which employs cyanide as a complexing agent has been used exclusively for many years in, for example, hydrometallurgical recovery of gold from auriferous ores. However, the serious impacts of cyanide toxicity upon waste disposal and upon the environment have made it urgent to reconsider the process.
It has been proposed to use thiourea, sodium thiosulfate, or the like in place of cyanide as the gold complexing agent. These substitutes, however, make the treatment so much more costly that they have seldom come into practical use.
Chlorine was tried earlier, but its adoption was given up because of its strong corrosive attack and high treatment cost involved.
Lixiviating a gold-containing material with iodine to recover gold is well known in the art. For example, processes for leaching gold from a gold-containing material with an iodine/iodide lixiviant or for recovering gold from a gold-iodine solution (commonly known as a pregnant lixiviant) directly obtained by leaching are described in U.S. Pat. Nos. 2,304,823, 3,957,505, and 4,557,759. Those processes entail the loss of expensive iodine and are not economically warranted.
A process for efficiently achieving both electrolytic recovery of gold and regeneration of iodine from an aqueous solution containing elemental iodine and iodide ions, taking advantage of the oxidizing power of iodine and the gold-complexing action of iodide ion has been established. Refer to PCT Patent Application Publication No. 502358/1988 (International Publication No. W087/03623) for details.
The process (hereinafter called the "iodine process") may be defined as: "A process for recovering gold by electrolysis from a pregnant, gold-bearing iodine lixiviant in which gold has been leached from a gold-containing material with an iodine/iodide lixiviant, while, at the same time, oxidizing part of the iodide ions in the lixiviant to regenerate iodine and recycle the lixiviant to the gold-leaching step."
To be more specific, the process is illustrated as comprising the steps of introducing the gold-bearing iodine lixiviant into the cathode compartment of an electrolytic cell, where gold is electrodeposited on the cathode electrode, reducing iodine in the lixiviant substantially to iodide, and conducting the effluent solution from the cathode compartment into the anode compartment, where the iodide ions are oxidized for regeneration to elemental iodine.
The iodine process is attracting attention as an excellent method for gold recovery to replace the cyanide process and offers the following advantages:
(1) It has fewer deleterious effects upon the environment. PA1 (2) Iodine in the lixiviant solution is stable in the form of a complex salt (I.sub.3.sup.-), and therefore, iodine loss during handling is minimized and the iodine concentration is easy to control. PA1 (3) The resulting gold complex salt is highly stable. PA1 (4) Regeneration of iodine permits recycling of the spent lixiviant solution, realizing low cost operation. PA1 (1) A process for eluting a gold-iodine complex from ion-exchange resins characterized by adding sulfuric acid and sodium nitrite to a gold-iodine complex adsorbed on a strongly basic anion-exchange resin, and thereafter, adding sodium sulfite thereto as an elutant; and PA1 (2) An elution process for gold-iodine complex from ion-exchange resins characterized by adding sulfuric acid and sodium nitrite to a gold-iodine complex adsorbed on a strongly basic anion-exchange resin, said complex being adsorbed beforehand by passage through the ion-exchange resin of washings that have resulted from the washing of the residue of a gold-containing material leached with an iodine/iodide lixiviant, and thereafter, adding sodium sulfite thereto as an elutant.
While the leaching process is, of course, essential for such gold recovery techniques, a washing system for the leach residue must also be considered. After the leaching of a gold-containing material and after the removal of the pregnant lixiviant, the leach residue usually contains about 20% of the iodine originally fed and a concomitantly formed gold-iodine complex. If the iodine-aided gold recovery process is to be commercially practicable, iodine and the gold-iodine complex in the residue should be recovered completely without any waste.
Recovery of gold from the gold-iodine solution (pregnant lixiviant) that results directly from the leaching of a gold-containing material is accomplished in a variety of ways. For example, as described in the aforementioned patents, a precipitate is formed using a reducing agent and then gold is recovered physically by filtration of the precipitate. Alternatively, gold is directly recovered electrochemically by an electrolytic process. However, the gold and iodine concentrations in the gold-iodine solution obtained by washing the gold-containing leach residue (this gold-iodine solution is generally called as a barren solution) are usually about one-tenth or less of the values of the pregnant lixiviant. Therefore, if some concentration operation is not done beforehand, gold cannot be efficiently recovered with the use of prior art processes.
As a means to concentrate gold from a gold-containing solution, the use of an ion-exchange resin has been proposed. For example, Gienn R. Palmer: "Ion-Exchange Research in Precious Metals Recovery," a publication published by the U.S. Bureau of Mines, deals with the recovery of a gold-cyanogen complex adsorbed on a strongly basic anion-exchange resin as associated with the cyanide process.
Previously, a removal of a gold complex by elution from a strongly basic anion-exchange resin on which it is adsorbed has involved considerable difficulty. One approach to recover gold from the complex is to allow the ion-exchange resin to adsorb gold up to saturation and bake the gold-adsorbing resin. The process, which involves incineration of the expensive ion-exchange resin, is economically disadvantaged. In addition, it poses a pollution problem due to the noxious gas evolution upon incineration. As an alternative, the adoption of a weakly basic (weak-base) anion-exchange resin which makes the elution easier has been proposed. In this case, it is necessary to add hydrogen ions to the weakly basic anion-exchange resin in advance to adsorb the gold complex on the resin. This limits the applicable pH range and the resin has a small adsorption capacity compared to the strongly basic anion-exchange resin. Another method of recovering gold with a chelate resin has been introduced. Disadvantages associated with the method are that the chelate resin, with a stronger adsorptive power than the strongly basic anion-exchange resin, makes elution practically impossible and that the resin itself is considerably more costly than ordinary ion-exchange resins.
The reference cited above describes the elution of a gold-cyanogen complex adsorbed on a strongly basic anion-exchange resin by the use of NaClO, NaCl, and NaOH. The reference process, thus directed to the elution of a cyanogen system, furnishes little information directed to the process of concern herein, which handles the iodine system.
To utilize a strongly basic anion-exchange resin that has a great capacity for adsorbing a gold-iodine complex in an iodine-aided gold recovery process, a novel method of eluting the gold-iodine complex effectively from the resin must be developed. This is particularly true when a lean gold-iodine complex is to be adsorbed for recovery from washings that contain the complex as a result of leaching of gold from a gold-containing material with the aid of iodine.